Gulls. Professor John C. Coulson

expectation of adult life and number of breeding years of different species ranging between four and 12 or more years. Annual adult survival rates for different species vary between 80 per cent and 92 per cent, and often vary appreciably from year to year and over longer periods of time. The annual survival rate tends to be higher in the larger gulls, which also have a longer period of immaturity. This delay in reaching breeding age in gulls appears to be associated with the time that is necessary to acquire competence in obtaining food, but why this should be longer in the large species is not evident. Ringed Herring Gulls that are nearly 35 years old have been recorded, but these represent a few extreme individuals comprising less than 1 per cent of those that reached maturity. In several gull species, the peak of mortality occurs during and just after the breeding season, when the adults are at their lowest weights during the year, suggesting that breeding is a significant stress. Data suggest that survival of gulls is usually high in winter, but the Kittiwake may be an exception, with most mortality occurring while the species is in its pelagic wintering range.

      Less is known of the survival of immature gulls, but there is usually a lower survival rate in the 12 months following fledging, after which the survival rate approaches that of the adults.

      The longevity records based on birds ringed as nestlings and living under natural conditions are given below, although several individuals are known to have lived longer in captivity.

Mediterranean Gull	22 years 1 month
Little Gull	20 years 11 months
Black-headed Gull	30 years 7 months
Common Gull	33 years 8 months
Lesser Black-backed Gull	34 years 10 months
European Herring Gull	34 years 9 months
Great Black-backed Gull	29 years 2 months
Black-legged Kittiwake	28 years 6 months
Ivory Gull	23 years 11 months
Laughing Gull	22 years 1 month
Ring-billed Gull	27 years 6 months
Glaucous-winged Gull	23 years 10 months

      The species with the longest recorded lifespans are mainly those that have been ringed in large numbers, and therefore have more chance of an exceptional record. It should be kept in mind that these lifespans are reached by exceptional individuals – perhaps one in several thousand – and so it is very likely that the maximum known age of many gull species in the wild will increase in future years as more recoveries of marked birds accumulate.

      SIZE DIFFERENCES BETWEEN SPECIES

      Gull species vary considerably in size. The Little Gull is the smallest and weighs about 100 g (the weight of an Arctic Tern, Sterna paradisaea), while the largest is the Great Black-backed Gull, with males averaging 1,800 g and some individuals exceeding 2,000 g. Fig. 4 shows the average weights of adult females of 19 gull species on the British list. The weights of females of nine of these species are less than 400 g on average and overlap with terns, of which the adult females of all except one species on the British breeding list weigh under 400 g. The distribution chart for male weights is similar, but is shifted to the right because of their slightly greater size.

      FIG 4. The average weight of the females of 19 gull species on the British list.

      INDIVIDUAL VARIATION

      Like all animals, individuals of each gull species show variation in many characters, including size, colour and age at first breeding. Males are larger than females and tend to have a more substantial bill, and size within a species can also vary geographically.

      Variation in the immature plumage is widespread in gulls of the same age and this is frequently overlooked in the field identification of species, particularly within the genus Larus. The occurrence of hybrid individuals further adds to plumage variation and typical examples of hybrids can often be identified in the field by experienced observers; the characteristics used often overlap with those of other species. Consequently a proportion of immature and even adult birds that are infrequently recorded in Britain and Ireland may fail to be identified because of potential confusion with other species.

      Immature plumages

      The plumage, leg and bill colours of recently fledged chicks are very different from those of their parents, to such an extent that, many years ago, a first-year Kittiwake was claimed as a species new to science, despite the adult having already been described and named some years earlier. The first plumage of the young of most gull species is made up of feathers of varying shades of brown and grey, producing a cryptic pattern that helps to conceal them in vegetation in a colony and also appears to reduce aggression from adults. In the smaller gull species, the plumage is replaced by one that resembles the adult at the first annual moult; that is, when the bird is 13 months or so old. In the larger species, all feathers are replaced each year, but only some of the new ones resemble those of the adults and the full adult plumage pattern is not achieved until four years after hatching. These progressive changes in plumage at successive annual moults can vary between individuals and produce a series of different plumage patterns that make it a challenge to identify both the species and the age of immature individuals.

      The slow and progressive acquisition of the adult plumage through successive moults contrasts with the rapid growth of bones and wing feathers, which reach full size within a few weeks after hatching because they are necessary before flight can be achieved. Why the acquisition of the adult plumage takes longer in the larger species of gull than in the smaller ones is not clear. The mechanism determining plumage patterns is obviously controlled by hormones and is linked with the greater length of immaturity in the larger gull species, but it is not evident why the large species delay reaching maturity for so long.

      Differences between the sexes

      The sexes of gulls have identical plumage and differ only in that females are usually smaller and tend to have slightly less substantial bills. Fig. 5 shows the relationship between the differences in weight of male and female gulls of seven species, using data from different parts of their geographical ranges where available. The extent of the difference between sexes is not constant from species to species, but increases with the weight of the species, ranging from a 5 per cent increase in the Black-headed Gull to more than 20–25 per cent in large species.

      Standard measurements of wing length also tend to be longer in males than in females, but the magnitude of the difference is much smaller, ranging from 1 per cent in some small species to 6 per cent in the largest species (Fig. 6). Even when this difference is converted to wing area, it still results in the wing loading being higher in the large gulls, which explains why these species typically have a more laboured flight, with a slower, more powerful wing-beat. The small gull species, which are of similar weight to many tern species, have a characteristic buoyant flight similar to that of terns.

      FIG 5. The percentage by which male gulls of several species are heavier than females, based on data for seven well-studied species. It is evident that there is a much greater difference between the size of males and females in the larger species of gulls.

      FIG